Transcoding method and apparatus therefor

ABSTRACT

A transcoding method and apparatus for converting a digital bitstream complying with a certain compression method into a digital bitstream complying with a different (or the same) compression method is provided. The transcoding apparatus performs conversion between compressed bitstreams having at least syntax elements and video elements corresponding to video data. The transcoding apparatus includes a decoder for reconstructing syntax elements and video elements from a first bitstream complying with a first compression method, an inverse quantizer for inverse-quantizing the video elements provided from the decoder according to the first compression method to reconstruct video data, a quantizer for requantizing the video data according to a second compression method, a syntax generator for mapping the syntax elements provided from the decoder to syntax elements complying with the second compression method, and an encoder for encoding the requantized video data (video elements complying with the second compression method) provided from the quantizer and the syntax elements provided from the syntax generator according to the second compression method, thereby outputting a second bitstream. The transcoding method facilitates the conversion between bitstreams complying with the same or different compression methods.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a transcoding apparatus forconverting a digital bitstream complying with a certain compressionmethod into a digital bitstream complying with a different (or the same)compression method, and more particularly, to a transcoding method forconverting a digital bitstream complying with a compression methodperformed using a quantizer without a dead zone into a digital bitstreamcomplying with a compression method using a quantizer with the deadzone, and an apparatus therefor.

[0003] 2. Description of the Related Art

[0004] The moving picture experts group (MPEG)-4 standard has beensettled down as a compression method essential for the applications ofrecent Internet videos, mobile videos and smart media. Meanwhile, mostexisting digital video contents have been compressed according to theMPEG-1 standard, and the MPEG-2 standard was selected as a fundamentalvideo compressing method for digital television broadcast (or highdefinition television (HDTV)). Therefore, it is highly expected that alarge number of digital video contents are compressed according to theMPEG-2 standard. Accordingly, to provide a lot of digital video contentsto a variety of terminals relating to digital videos afterwards, it willbe important to converting digital video contents encoded according tothe existing MPEG-1 or MPEG-2 standard into digital video contentscomplying with the MPEG-4 standard.

SUMMARY OF THE INVENTION

[0005] To solve the above problems, it is a first object of the presentinvention to provide a method of converting digital video contentscompressed according to the moving picture experts group (MPEG)-1standard or the MPEG-2 standard into digital video contents complyingwith the MPEG-4 standard.

[0006] It is a second object of the present invention to provide amethod of converting a discrete cosine transform (DCT) coefficientsquantized according to quantization without a dead zone into quantizedDCT coefficients complying with quantization with the dead zone.

[0007] It is a third object of the present invention to provide anapparatus suitable for the above methods.

[0008] Accordingly, to achieve the first object of the invention, thereis provided a transcoding method of performing conversion betweencompressed bitstreams having at least syntax elements and video elementscorresponding to video data. The transcoding method includes the stepsof a) decoding a first bitstream compressed according to a firstcompression method and parsing syntax elements and video elements; b)mapping the parsed syntax elements to syntax elements complying with atarget second compression method; c) partially reconstructing video datacomplying with the first compression method from the parsed videoelements; d) requantizing the video data reconstructed in the step c)according to the second compression method; and e) coding the mappedsyntax elements and the requantized video data to obtain a bitstreamcomplying with the second compression method.

[0009] To achieve the second object of the invention, there is provideda requantizing method in which an output y with respect to an input DCTcoefficient x is expressed by${y = {{Q_{1}(x)} = \left\lfloor {\left\lfloor {\frac{x}{\Delta} + \frac{1}{2}} \right\rfloor \cdot \Delta} \right\rfloor}},$

[0010] a quantization step size Δ_(i) is given by${{\Delta \quad i} = \frac{{Wi} \cdot Q_{p}}{8}},{i = 0},1,{2\quad \ldots},63$

[0011] (Q_(p) is a quantization parameter), a decision level t_(m) isgiven by${t_{m} = {\left( {m - \frac{1}{2}} \right) \cdot \Delta}},\quad {m \geq 1},{x_{m} = \left\{ {x\left. {x \in \left\lbrack {t_{m,}t_{m + 1}} \right\rbrack} \right\}} \right.}$

[0012] when x belongs to a section [t_(m), t_(m+1)], an amplitude levelλ_(m) of x_(m) is expressed by${\lambda_{m} = \left\lfloor {\frac{x_{m}}{\Delta} + \frac{1}{2}} \right\rfloor},$

[0013] an output x′ with respect to the input DCT coefficient y, whichhas been quantized by a MPEG-1 quantizer having a dead zone in which areconstruction level for x_(m), that is, an inverse-quantized DCTcoefficient r_(m) is given by r_(m)=└λ_(m)·Δ┘, is expressed by$x^{\prime} = {{Q_{2}(y)} = \left\{ \begin{matrix}{\quad \left\lfloor {{\left\lfloor \frac{y}{\Delta^{\prime}} \right\rfloor \cdot \Delta^{\prime}} + \frac{\Delta^{\prime}}{2}} \right\rfloor} & {\quad {{if}\quad Q_{p}\quad {is}\quad {odd}}} \\{\quad {\left\lfloor {{\left\lfloor \frac{y}{\Delta^{\prime}} \right\rfloor \cdot \Delta^{\prime}} + \frac{\Delta^{\prime}}{2}} \right\rfloor - 1}} & {\quad {{if}\quad Q_{p}\quad {is}\quad {even}}}\end{matrix} \right.}$

[0014] a quantization step size Δ′ is given by Δ′=2Q_(p), a decisionlevel t′_(n) is given by t′_(n)=n·Δ′, n≦1, y_(n)={y|yε[t′_(n),t′_(n+1),]} when the output y belongs to a section [t′_(n),t′₁₊₁], and an amplitude level of y_(n), that is, an inverse-quantizedDCT coefficient λ′_(n) is requantized by a MPEG-4 quantizer having adead zone defined as$\lambda_{n}^{\prime} = \left\lfloor \frac{y_{n}}{\Delta^{\prime}} \right\rfloor$

[0015] and is converted into a MPEG-4 DCT coefficient. The requantizingmethod includes the steps of d-1) defining subscript values allowing thedecision level to belong to a section [t_(m), t_(m+1)] as a setp={p|t′_(p)ε[t_(m), t_(m+1)]}; d-2) defining candidates of the subscriptvalues of the decision level as a set K=P∪{min{P}−1} where the symbol ∪indicates a union and an operator min{A} indicates a minimum value amongthe members of a set A; and d-3) selecting a member satisfying a costfunction from among the candidate subscript values as a final subscriptvalue, the cost function being expressed by $k = {{\arg \begin{matrix}{\min \quad} \\{k \in K}\end{matrix}{{C_{m} - r_{k}^{\prime}}}\quad {where}\quad C_{m}} = \frac{\int_{t_{m}}^{t_{m} + 1}{{x \cdot ~{p(x)}}{x}}}{\int_{t_{m}}^{t_{m} + 1}{{p(x)}{x}}}}$

[0016] where C_(m) is an optimum reconstruction level in the section[t_(m), t_(m+1)] used by a Lloyd-Max quantizer in view of mean squareerror, and p(x) is a Laplacian distribution function.

[0017] To achieve the third object of the invention, there is provided atranscoding apparatus of performing conversion between compressedbitstreams having at least syntax elements and video elementscorresponding to video data. The transcoding apparatus includes adecoder for reconstructing syntax elements and video elements from afirst bitstream complying with a first compression method; an inversequantizer for inverse-quantizing the video elements provided from thedecoder according to the first compression method to reconstruct videodata; a quantizer for requantizing the video data according to a secondcompression method; a syntax generator for mapping the syntax elementsprovided from the decoder to syntax elements complying with the secondcompression method; and an encoder for encoding the requantized videodata (video elements complying with the second compression method)provided from the quantizer and the syntax elements provided from thesyntax generator according to the second compression method, therebyoutputting a second bitstream.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The above objectives and advantages of the present invention willbecome more apparent by describing in detail preferred embodimentsthereof with reference to the attached drawings in which:

[0019]FIG. 1 is a diagram illustrating the concept of a transcodingapparatus which is applied to moving picture experts group (MPEG)-1compression;

[0020]FIGS. 2 and 3 are diagrams illustrating conventional embodimentsof the transcoding apparatus of FIG. 1;

[0021]FIG. 4 is a diagram illustrating the processes of a transcodingmethod according to the present invention, through which a MPEG-1bitstream is converted into a MPEG-4 bitstream;

[0022]FIG. 5 is a flowchart illustrating a method of requatizing DCTcoefficients according to the present invention;

[0023]FIGS. 6A through 6E are diagrams for explaining the concept ofquantization according to the present invention;

[0024]FIGS. 7A and 7B are diagrams for explaining requantizationaccording to the present invention;

[0025]FIGS. 8A and 8B are graphs illustrating the effects ofrequantization according to the present invention;

[0026]FIGS. 9A through 9C illustrate an original image, an imagecorresponding to bitstreams transcoded according to a conventionalmethod and an image corresponding to bitstreams transcoded according tothe present invention, respectively;

[0027]FIGS. 10 through 12 are graphs illustrating the results of testinga transcoding method according to the present invention; and

[0028]FIG. 13 is a block diagram illustrating a transcoding apparatusaccording to the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0029]FIG. 1 illustrates the example of a transcoding apparatus appliedto moving picture experts group (MPEG)-1 compression. The transcodingapparatus of FIG. 1 converts an input MPEG-1 bitstream into a MPEG-1 bitstream having a different aspect ratio or a different bit rate. Here,the MPEG-1 bitstream indicates a digital video bitstream which has beencompressed according to MPEG-1 compression.

[0030] A transcoder 100 includes an internal MPEG-1 decoder 104 and aninternal MPEG-1 encoder 106. The internal MPEG-1 decoder 104 decodes aMPEG-1 bitstream provided from an external MPEG-1 encoder 102 to formvideo data. The internal MPEG-1 encoder 106 encodes the video datareconstructed by the internal MPEG-1 decoder 104 to form a MPEG-1bitstream having a different aspect ratio or a different bit rate. Anexternal MPEG-1 decoder 108 decodes the MPEG-1 bitstream output from theinternal MPEG-1 encoder 106 to form video data. Due to the operation ofthe transcoder 100, the aspect ratio and the bit rate of a MPEG-1bitstream input to the transcoder 100 are different from those of aMPEG-1 bitstream output therefrom.

[0031] The external MPEG-1 encoder 102 may be installed in a programprovider party such as a video-on-demand (VoD) service provider, and theexternal MPEG-1 decoder 108 may be installed in a receiving partyprovided with programs by the VoD service provider. In other words, thetranscoder 100 converts bitstreams provided by a program provider intobitstreams having a bit rate and an aspect ratio suitable for a displayunit provided in a receiving party.

[0032]FIGS. 2 and 3 are diagrams illustrating conventional embodimentsof the transcoding apparatus of FIG. 1. FIGS. 2 and 3 illustrates anopen loop transcoder and a closed loop transcoder, respectively. Here,the open loop transcoder reconstruct only a portion, for example, amacroblock (MB), of video data from a bitstream. On the other hand, theclosed loop transcoder reconstructs entire video data from a bitstreamand encodes the video data. In MPEG compression, a MB is typicallycomposed of 4 luminance blocks and 2 color difference blocks.

[0033] Since input and output bitstreams are compressed by the samemethod or have the same syntax, the explanation of the operations of theapparatus of FIGS. 2 and 3 is restricted to conversion of videoelements, in particular, to conversion of MBs. Each MB includes adiscrete cosine transform (DCT) coefficient encoded according to avariable length coding method, a motion vector and MB information.

[0034] The open loop transcoder of FIG. 2 performs conversion in MBunits. The motion vector and the MB information of each MB are separatedfrom the MB before conversion and are united to the MB after theconversion. The DCT coefficient of the MB is decoded by a variablelength decoder (VLD) 202 and then inverse-quantized by an inversequantizer (IQ1) 204. The IQ1 204 recovers a DCT coefficient of each DCTblock. A quantizer (Q2) 206 quantizes the DCT coefficient of each DCTblock recovered by the IQ1 204 to be suitable for a set bit rate. Avariable length coder (VLC) 208 variable length codes the DCTcoefficient quantized by the Q2 206. A DCT coefficient output from theVLC 208 is united with the motion vector and the MB information whichhave been separated before conversion.

[0035] The apparatus of FIG. 3 reconstructs entire video data from aMPEG-1 bitstream and re-encodes the video data to have a desired aspectratio and bit rate. A decoder 300 reconstructs video data from an inputMPEG-1 bitstream. The reconstructed video data is re-encoded by anencoder 320. A bitstream corresponding to a desired aspect ratio and bitrate can be obtained due to the operation of the encoder 320. Thedecoder 300 is a typical MPEG-1 decoder and includes an inversequantizer (IQ1) 302, an inverse DCT converter (IDCT1) 304, an adder 306,a memory (MEM1) 308 and a motion compensator (MC1) 310. The encoder 320is a typical MPEG-1 encoder and includes a subtractor 322, a DCTconverter (DCT2) 324, a quantizer (Q2) 326, an inverse quantizer (IQ2)328, an inverse DCT converter (IDCT2) 330, a subtractor 332, a memory(MEM2) 334 and a motion compensator (MC2) 336.

[0036] An open loop transcoder as shown in FIG. 2 demonstrates theexcellent performance with a very small complexity when a video codingmethod is an intra-picture (I-picture) coding method. However, when avideo coding method is an inter-picture (P-picture or B-picture) codingmethod, the open loop transcoder has problems in that a reference imagechanges due to a quantization error changing during motion compensation,resulting in the accumulation of errors (referred to as a drift).Consequently, a picture quality is degraded.

[0037] A closed loop transcoder as shown in FIG. 3 is faithful to theconcept illustrated in FIG. 1. An entire bitstream is decoded toreconstruct video data (by the decoder 300), and then the video data isre-encoded to generate a bitstream having a desired format (by theencoder 320). Such a closed loop transcoder has an advantage of stablygenerating a bitstreams of a desired bit rate, but has disadvantages ofa large complexity of the apparatus and a large amount of calculation.

[0038] In addition, the transcoders of FIGS. 2 and 3 perform aconversion between the same encoding methods so that they cannot beapplied to MPEG-4 terminals which are essential to a variety ofmultimedia communications and storage relating to Internet videos,mobile networks, smart media and the like.

[0039] To overcome these problems, the present invention provides anapparatus and method for converting a bitstream generated according tothe MPEG-1 or MPEG-2 standard into a bitstream generated according tothe MPEG-4 standard. In addition, the present invention provides anoptimum requantization method for maintaining an image quality of muchas possible during requantization of a DCT coefficient.

[0040] Transcoding according to the present invention is performed asfollows.

[0041] a) An input MPEG-1 (or MPEG-2) bitstream is variable lengthdecoded, and a MPEG-1 (or MPEG-2) syntax element and a DCT coefficientis parsed and extracted from the input bitstream. In this specification,description of this process is restricted to a video syntax element andDCT coefficient.

[0042] b) The extracted MPEG-1 (or MPEG-2) syntax element is mapped tobe suitable for the MPEG-4 standard.

[0043] c) The extracted DCT coefficient is inverse-quantized accordingto the MPEG-1 (or MPEG-2) standard.

[0044] d) An inverse-quantized DCT coefficient is requantized accordingto the MPEG-4 standard to meet a desired bit rate. A bit rate is setduring this requantization.

[0045] e) The syntax element mapped to comply with the MPEG-4 standardand the DCT coefficient requantized according to the MPEG4 standard arevariable length coded, thereby obtaining a bitstream complying with theMPEG-4 standard.

[0046]FIG. 4 is a diagram illustrating the processes of a transcodingmethod according to the present invention, through which a MPEG-1bitstream is converted into a MPEG-4 bitstream. In FIG. 4, the first rowshows detailed processes for the parsing of MPEG-1 syntax elements andprocesses of inverse-quantizing a DCT coefficient. The second row showsprocesses of mapping the MPEG-1 syntax elements to MPEG-4 syntaxelements. The third row shows processes of forming a MPEG-4 bitstreamfrom the MPEG-4 syntax elements and a quantized DCT coefficient obtainedthrough requantization.

[0047] In step 402, a sequence header is parsed. In step 404, a group ofpicture (GOP) header is parsed. In step 406, a picture header is parsed.In step 408, a slice header is parsed. In step 410, a MB header isparsed. In the steps 402 through 410, the sequence header, the GOPheader, the picture header, the slice header and the MB header aresequentially separated from an input MPEG-1 bitstream. In the MPEG-1(MPEG-2) standard, an image has a layer structure composed of a DCTblock, a MB, a slice, a picture and a GOP in the ascending order ofsize. In step 412 of converting an MB to a DCT, a DCT coefficient isreconstructed in MB unit.

[0048] In an object layer syntax element mapping step 420, the syntaxelements of a MPEG-1 sequence layer are mapped to syntax elementsvopwidth, vop_height, bit_rate, fixed_vop_rate and aspect_ratio_info ofa MPEG-4 sequence layer based on information contained in the parsedMPEG-1 sequence header obtained by performing the step 402. In an objectplane syntax element mapping step 422, the syntax elements of a MPEG-1object plane layer are mapped to syntax elements vop_coding_type,vop_fcode and vop_quant of a MPEG-4 object plane layer based oninformation contained in the parsed MPEG-1 picture header obtained byperforming the step 406. In a slice layer element mapping step 424, thesyntax element of a MPEG-1 slice layer is mapped to a syntax elementslicequant of a MPEG-4 slice layer based on information contained in theparsed MPEG-1 slice header obtained by performing the step 408. In a MBlayer element mapping step 426, the syntax elements of a MPEG-1 MB layerare mapped to syntax elements COD, DQUANT, CBPC, CBPY and MV of a MPEG-4MB layer based on information contained in the parsed MB header obtainedby performing the step 410. In a requantization step 428, the DCTcoefficient of the MPEG-1 standard, which is obtained by performing thestep 412, is requantized according to the MPEG-4 standard.

[0049] In an object header generating step 430, an object headerincluding the MPEG-4 object layer syntax elements vop_width, vop_height,bit_rate, fixed_vop rate and aspect_ratio_info obtained through the step420 is generated. In an object plane header generating step 432, anobject plane header including the MPEG-4 object plane syntax elementsvop_coding_type, vop_fcode and vop_quant obtained through the step 422is generated. In a MB header generating step 434, a MB header includingthe MPEG-4 MB layer syntax elements COD, DQUANT, CBPC, CBPY and MVobtained through the step 426 is generated. In an encoding step 436, therequantized DCT coefficient obtained through the step 428, the objectheader generated in the step 430, the object plane header generated inthe step 432 and the MB header generated in the step 434 are united andvariable length coded thereby generating a MPEG-4 bitstream.

[0050] In the present invention, a procedure of mapping MPEG-1 (orMPEG-2) syntax elements to MPEG-4 syntax elements is as follows.

[0051] b-1) Since a syntax element f_code (indicating the motion rangeof a motion vector) in MPEG-1 has a different meaning from that inMPEG-4, the MPEG-1 f_code is converted into a MPEG-4 f_code. In Table 1,the motion ranges indicated rs and codes in MPEG-1 and MPEG-4 arecompared. TABLE 1 MPEG-4 MPEG-1 vop_f_(—) Motion forward_(—) Motioncode_forward range f_code range 1 [−32, 31] 1 [−16, 15] 2 [−64, 63] 2[−32, 31] 3 [−128, 127] 3 [−64, 63] 4 [−256, 255] 4 [−128, 127] 5 [−512,511] 5 [−256, 255] 6 [−1024, 1023] 6 [−512, 511] 7 [−2048, 2047] 7[−1024, 1023]

[0052] As shown in Table 1, in the same motion range, the value of theMPEG-4 vop_f_code forward is smaller than the value of the MPEG-1forward f code by 1. In addition, a motion range corresponding to theminimum value 1 of the MPEG-4 vop_f_code_forward covers a motion rangecorresponding to the minimum value 1 of the MPEG-1 forward f code. Thiscan be expressed by Equation (1).

vop _(—) f_code_forward=max((forward_(—) f_code-1),1)  (1)

[0053] where max(a, b) is an operator of selecting a larger valuebetween a and b.

[0054] b-2) A MPEG-1 MB type is converted into a MPEG-4 MB type.

[0055] Table 2 compares the MB types between the MPEG-1 standard and theMPEG-4 standard. MPEG-1 MB MPEG-4 MB motion MB VOP MB Not Picture MB MBmotion back- motion MB type type Name coded mcbpc cbpy dquant mvd mvd2-4type type quant forward ward pattern intra P Not — 1 I intra 0 0 0 0 1coded P 0 inter 1 1 1 1 1 I intra + 1 0 0 0 1 q P 1 inter + 1 1 1 1 PMC + 0 1 0 1 0 q coded P 2 inter + 1 1 1 1 1 1 P nomc + 0 0 0 1 0 4 Vcoded P 3 intra 1 1 1 P mc + 0 1 0 0 0 not coded P 4 intra + 1 1 1 Pintra 0 0 0 0 1 q P stuff- — 1 1 P mc + 1 1 0 1 0 ing coded + q I 3intra 1 1 P nomc + 1 0 0 1 0 coded + q P intra + 1 0 0 0 1 q

[0056] As shown in Table 2, there are 6 MB types except “stuffing” and“inter+4v” in MPEG-4, and there are 8 MB types in MPEG-1.

[0057] The three MB types “nomc+coded”, “nomc+coded+q” and “mc+notcoded” among the MB types in MPEG-1 do not exist in MPEG-4. To realizethese properties, conversion is performed according to the followingrules.

[0058] (i) The MB type “nomc+coded” is set as the “inter” type ofMPEG-4, and then a motion vector is set to (0, 0).

[0059] (ii) The MB type “nomc+coded+q” is set as the “inter+q” type ofMPEG-4, and then a motion vector is set to (0, 0).

[0060] (iii) The MB type “mc+not coded” is set as the “inter” type ofMPEG-4. A motion vector is used as it is. The values of “cbpy” and“cbpc” are set to zero.

[0061] (iv) For skipped MBs, the value of “code” determining “not coded”in MPEG-4 is set to 0 such as “cod=0” as many times as the skipped MBs.

[0062] b-3) A MPEG-1 CBP is converted into MPEG-4 CBP.

[0063] Two kinds of information cbpy (coded block pattern of Y where Yis a luminance signal) and cbpc (coded block pattern of C where C is acolor signal) in MPEG-4 are united in one syntax element cbp in MPEG-1.The syntax element cbp in MPEG-1 indicates the existence/non-existenceof a DCT block including a non-zero DCT coefficient for the DCT blocksincluded in a MB, and has as many bits as the DCT blocks included in theMB. Each bit indicates whether there is any non-zero DCT coefficient ina corresponding DCT block. Typically, a MB is composed of 4 luminanceDCT blocks and 2 color difference DCT blocks so that the syntax elementcbp is represented with 6 bits.

[0064] The syntax element cbpy in MPEG-4 corresponds to bits (4 bits)corresponding to the luminance DCT blocks in the MPEG-1 cbp, and thesyntax element cbpc in MPEG-4 corresponds to bits (2 bits) correspondingto the color difference DCT blocks in the MPEG-1 cbp.

[0065] The syntax element cbpy is individually coded, and this can beexpressed by Equation (2).

cbpy=(cbp&0x03c)>>2  (2)

[0066] where “&” indicates an AND operation performed in bit unit,“0×3c” indicates “3c” of a hexadecimal number, and “>>n” indicates ann-bit right shift operation.

[0067] The syntax element cbpc can be expressed by Equation (3).

cbpc=(cbp&0x03)>>2  (3)

[0068] The syntax element cbpc is united with the MB type obtained inthe above step (b-2) and coded to comply with the mcbpc VLC table ofcorresponding MPEG-4 I-VOP and P-VOP.

[0069] b-4) The value of a MPEG-1 MQUANT is converted into a value of aMPEG-4 DQUANT.

[0070] The value of the MPEG-4 DQUANT is the difference between adjacentMBs in a quantization parameter (QUANT). QUANT values exist in the rangeof [1, 31] and should satisfy a relation, |Q_(n)−Q_(n−1)|≦2. Convertinga MPEG-1 MQUANT value into a MPEG-4 DQUANT value can be expressed byEquation (4).

dquant=min (max((mquant of current MB−mquant of previous MB),−2),2)  (4)

[0071] where “mquant” indicates a MPEG-1 quantization parameter.

[0072] When fully reflecting the range of a current mquant to ensureaccuracy much more, a resynchronization maker according to the MPEG-4standard can be used.

[0073] In MPEG-1 and MPEG-4, a quantization step size varies with aquantization weight as well as a quantization parameter. Usually, aMPEG-1 quantization step size Δ is always larger than a MPEG-4quantization step size Δ′. Accordingly, when requantizing a DCTcoefficient reconstructed from a MPEG-1 bitstream according to theMPEG-4 standard, disagreement between MPEG-1 and MPEG-4 in aquantization size should be appropriately handled to maximize thequality of an image reconstructed from a converted MPEG-4 bitstream.

[0074]FIG. 5 is a flowchart illustrating a method of requatizing DCTcoefficients according to the present invention. A procedure ofrequantizing a DCT coefficient in a transcoding method according to thepresent invention will be described with reference to FIG. 5. In step502, a DCT coefficient is reconstructed from a MPEG-1 bitstream in MBunit. In step 504, the Laplacian distribution of the DCT coefficient isestimated. In step 506, a reconstruction level is determined using theestimated Laplacian distribution characteristics of the DCT coefficient.In step 508, quantization is performed according to MPEG-4 using thedetermined reconstruction level.

[0075]FIGS. 6A through 6E are diagrams for explaining the concept ofquantization according to the present invention. FIG. 6A illustrates aMPEG-1 quantizer. FIG. 6B illustrates quantization weighting factors Wiof a fundamental quantization table for luminance component used inquantizing an intra MB. Based on the quantization weighting factors ofthe quantization table, a quantization step size is determined asfollows. $\begin{matrix}{{\Delta \quad i\frac{{Wi} \cdot Q_{p}}{8}},{i = 0},1,{2\quad \ldots},63} & (5)\end{matrix}$

[0076] where Q_(p) is a quantization parameter.

[0077] In FIG. 6A, the relation between the input x and output y of thequantizer with respect to an arbitrary DCT coefficient can be expressedas follows. $\begin{matrix}{y = {{Q_{1}(x)} = \left\lfloor {\left\lfloor {\frac{x}{\Delta} + \frac{1}{2}} \right\rfloor \cdot \Delta} \right\rfloor}} & (6)\end{matrix}$

[0078] where an operator └a┘ indicates an integer most approximate to“a”.

[0079] A decision level t_(m) is given by Equation (7). $\begin{matrix}{{t_{m} = {\left( {m - \frac{1}{2}} \right) \cdot \Delta}},\quad {m \geq 1}} & (7)\end{matrix}$

[0080] When x belongs to a range [t_(m), t_(m+1)], x_(m)={x|ε[t_(m),t_(m+1)]}. An amplitude level λ_(m) of x_(m) is given by Equation (8).$\begin{matrix}{\lambda_{m} = \left\lfloor {\frac{x_{m}}{\Delta} + \frac{1}{2}} \right\rfloor} & (8)\end{matrix}$

[0081] From Equations (8) and (6), x_(m) is mapped to an m-threconstruction level according to Equation (9).

r _(m)=└λ_(m)·Δ┘  (9)

[0082] An input x having a negative value has the following quantizationmapping relation.

y=−Q ₁(|x|)  (10)

[0083]FIG. 6C illustrates the quantizer characteristics for a MPEG-1intra MB. In FIG. 6C, a unit on the vertical and horizontal axes “x” and“y” is a quantization step size, A. Here, a MPEG-1 quantizer is auniform quantizer and does not have a dead zone. In other words, thelength of a section including the origin is the same as that of anyother section on the vertical axis.

[0084]FIG. 6D illustrates a MPEG-4 quantizer. The output x′ of theMPEG-4 quantizer with respect to an input y is given by Equation (11).$\begin{matrix}{x^{\prime} = {{Q_{2}(y)} = \left\{ \begin{matrix}\left\lfloor {{\left\lfloor \frac{y}{\Delta^{\prime}} \right\rfloor \cdot \Delta^{\prime}} + \frac{\Delta^{\prime}}{2}} \right\rfloor & {{if}\quad Q_{p}\quad {is}\quad {odd}} \\{\left\lfloor {{\left\lfloor \frac{y}{\Delta^{\prime}} \right\rfloor \cdot \Delta^{\prime}} + \frac{\Delta^{\prime}}{2}} \right\rfloor - 1} & {{if}\quad Q_{p}\quad {is}\quad {even}}\end{matrix} \right.}} & (11)\end{matrix}$

[0085] A quantization step size Δ′ is given by Equation (12).

Δ′=2Q _(p)  (12)

[0086] where Q_(p) is a quantization parameter.

[0087] A decision level t′_(n) is given by Equation (13).

t′ _(n) =n·Δ′, n≧1  (13)

[0088] When the output y belongs to a range [t′_(n), t′_(n+1) ], y _(n)={y|y ε[t′ _(n) , t′ _(n+1)]}. An amplitude level λ′_(n) of y_(n) isgiven by Equation (14). $\begin{matrix}{\lambda_{n}^{\prime} = \left\lfloor \frac{y_{n}}{\Delta^{\prime}} \right\rfloor} & (14)\end{matrix}$

[0089] The characteristics of the MPEG-4 quantizer are shown in FIG. 6E.In FIG. 6E, the existence of a dead zone is confirmed. In other words,the length of a section including the origin is different from that ofany other section on the vertical axis.

[0090] In Equation (5), a MPEG-1 quantization step size Δ is weightedwith a quantization table value Wi in addition to a quantizationparameter Q_(p). On the other hand, in the case of MPEG-4, thequantization parameter Q_(p) is doubled according to Equation (12), andthe resultant value is set as a quantization step size Δ′.

[0091] Accordingly, except the case where the quantization table valueWi is 16, a MPEG-1 quantization step size Δ is always larger than aMPEG-4 quantization step size Δ′. Referring to FIG. 6B, only when aquantization index i is 1, 8 or 9, a MPEG-1 quantization step size isthe same as a MPEG-4 quantization step size. Although a MPEG-1quantization step size is the same as a MPEG-4 quantization step sizewhen a quantization index i is 1, 8 or 9, the dead zone of the quantizerQ1 is not the same as that of the quantizer Q2, as shown in FIGS. 6C and6E, so that it can be appreciated that the two quantizers have differentcharacteristics.

[0092]FIGS. 7A and 7B are diagrams for explaining requantizationaccording to the present invention. FIG. 7A illustrates a consecutivequantization structure in which the quantizers Q1 and Q2 of FIGS. 6A and6D are combined in order to show requantization which should beconsidered for a transcoder. FIG. 7B illustrates the quantizationcharacteristics of the quantizers Q1 and Q2 under a usual state where aMPEG-1 quantization step size Δ is larger than a MPEG-4 quantizationstep size Δ′. When an original DCT coefficient x is [t_(m), t_(m+1)],y=r_(m) in FIG. 7B. When this value is input to the quantizer Q2, r′_(n)is reconstructed as the value of x′. Here, as shown in FIG. 7B, thereare three candidate values r′_(n−1),r′_(n) and r′_(n+1) of areconstruction level, which can be taken by the quantizer Q2. In otherwords, distortion can be reduced by the quantizer Q2 selecting anoptimum reconstruction level.

[0093] As described above, due to the disagreement between thequantizers Q1 and Q2 in a dead zone and a quantization step size, thequantizer Q2 has a plurality of candidates of an amplitude level andneeds to determine an optimum amplitude level among the candidates. Anoptimum amplitude level minimizes distortion and the number of generatedbits. To determine such an optimum amplitude level, the presentinvention suggests an optimum amplitude level selection algorithm asfollows.

[0094] Subscript values allowing a decision level to belong to a section[t_(m), t_(m+1)] in FIG. 7B are defined by a set P as shown below.

P={p|t′ _(p) ε[t _(m+1),]}  (15)

[0095] The candidates of the subscript values of the decision level aredefined by a set K as follows.

K=P∪{min{P}−1}  (16)

[0096] where the symbol ∪ indicates a union, and an operator min{A}indicates a minimum value among the members of a set A.

[0097] Among the candidate subscript values, the members of the set K, asubscript value satisfying the following cost function is selected.$\begin{matrix}{k = {\left. \arg_{k \in K}^{\min} \middle| {C_{m} - r_{k}^{\prime}} \middle| \quad {{where}\quad C_{m}} \right. = \frac{\int_{t_{m}}^{t_{m} + 1}{{x \cdot {p(x)}}{x}}}{\int_{t_{m}}^{t_{m} + 1}{{p(x)}{x}}}}} & (17)\end{matrix}$

[0098] A amplitude level λ′ corresponding to the subscript k determinedin accordance with the cost function expressed by Equation (17) isvariable length coded. A balance point C_(m) in the cost function ofEquation (17) is an optimum reconstruction level in a section [t_(m),t_(m+1)] used by a Lloyd-Max quantizer in view of mean square error.Accordingly, the balance point C_(m) can be calculated from thedistribution p(x) of x.

[0099] However, a transcoder cannot obtain the accurate statisticalcharacteristic of the distribution p(x). To solve this problem, it isnecessary to previously calculate the statistical characteristic of anoriginal DCT coefficient when producing a MPEG-1 bitstream and transmitthe information with the MPEG-1 bitstream to a transcoder.Alternatively, it is necessary to design a transcoder such that it canestimate the statistical characteristic of a distribution p(x). Theformer method is disadvantageous in increasing the number of overheadbits for additional information. The present invention uses the lattermethod.

[0100] It is generally known that AC DCT coefficients comply with thefollowing Laplacian distribution. $\begin{matrix}{{p(x)} = {\frac{\lambda}{2} \cdot e^{{- \lambda}|x|}}} & (18)\end{matrix}$

[0101] Here, the statistical characteristic of a distribution p(x) isdetermined by the value of λ. In each block, different values of λ areassigned to 63 AC coefficients.

[0102] When using Equation (18), an average of a random variable |x| canbe given by Equation (19). $\begin{matrix}{{{E\left( |x| \right)} = {\int_{- \infty}^{\infty}|x|}}{{{\cdot {p(x)}}{x}} = {\int_{- \infty}^{\infty}|x|}}{{{\cdot \frac{\lambda}{2} \cdot e^{{- \lambda}|x|}}{x}} = \frac{1}{\lambda}}} & (19)\end{matrix}$

[0103] From Equation (19), λ can be expressed by the following equation.$\begin{matrix}{\lambda = \frac{1}{E\left( |x| \right)}} & (20)\end{matrix}$

[0104] To verify Equation (20), λ for a first frame of a flower gardensequence according to SIF standards was estimated using Equation (20),and the results are shown in Table 3. TABLE 3 0 1 2 3 4 5 6 7 0 — 0.0190.027 0.040 0.057 0.076 0.107 0.170 1 0.018 0.027 0.035 0.046 0.0630.088 0.133 0.208 2 0.021 0.029 0.037 0.049 0.066 0.092 0.133 0.220 30.025 0.033 0.040 0.050 0.066 0.094 0.148 0.255 4 0.028 0.034 0.0440.056 0.075 0.107 0.160 0.267 5 0.032 0.037 0.046 0.058 0.078 0.1150.166 0.293 6 0.036 0.041 0.048 0.062 0.089 0.123 0.173 0.304 7 0.0430.047 0.058 0.069 0.093 0.131 0.188 0.313

[0105]FIGS. 8A and 8B are graphs illustrating the accuracy in estimatingthe value of λ according to the present invention. To confirm theresults shown in Table 3, a Laplacian distribution when λ=0.046 iscompared with an actual distribution for a coefficient (1, 3) in FIG.8A, and a Laplacian distribution when λ=0.166 is compared with an actualdistribution for a coefficient (5, 6) in FIG. 8B. It can be seen thatthe value of λ is smaller as an AC component becomes lower in frequency.This is because the probability that DCT coefficients having largervalues exist at lower frequency is high due to the energy packingcharacteristics of DCT.

[0106] Although the value of λ can be calculated from Equation (19), thevalue of x cannot be obtained, but only the value of a reconstructed ycan be obtained in a transcoder. Accordingly, it is necessary toapproximate the value of E(|x|) according to Equation (21).

E(|x|)≡E(|y|)+E(|z|),_(Δ/2)  (21)

[0107] The second term in Equation (21) is added to compensate for anaverage of the values of |x|, which are reconstructed to be zero afterquantization because they belong to a dead zone, and is defined asEquation (22). $\begin{matrix}{{E\left( |z| \right)}_{\frac{\Delta}{2}} = {\int_{- \frac{\Delta}{2}}^{\frac{\Delta}{2}}\left| z \middle| {{\cdot {p(z)}}{z}} \right.}} & (22)\end{matrix}$

[0108] The value of p(z) necessary for calculating the second term inEquation (21) is given by Equation (23). $\begin{matrix}{{p(z)} = {{{\frac{\lambda^{\prime}}{2} \cdot e^{{- \lambda^{\prime}}|z|}}\quad {where}\quad \lambda^{\prime}} = \frac{1}{E\left( |y| \right)}}} & (23)\end{matrix}$

[0109] E(|z|)_(Δ/2) in Equation (23) is calculated according to Equation(24). $\begin{matrix}{{E\left( |z| \right)}_{\frac{\Delta}{2}} = {{2 \cdot {\int_{0}^{\frac{\Delta}{2}}{{z \cdot \frac{\lambda^{\prime}}{2} \cdot e^{{- \lambda^{\prime}}/z}}{z}}}} = {\frac{1}{\lambda^{\prime}} - {e^{{- \lambda^{\prime}}{\Delta/2}}\left( {\frac{1}{\lambda^{\prime}} + \frac{\Delta}{2}} \right)}}}} & (24)\end{matrix}$

[0110] Accordingly, the value of λ estimated by a transcoder accordingto the present invention is expressed as follows. $\begin{matrix}{\lambda = {{\frac{1}{E\left( {x} \right)} \cong \frac{1}{{E\left( {y} \right)} + {E\left( {z} \right)}_{\frac{\Delta}{2}}}} = \frac{\lambda^{\prime}}{2 - {^{{- \lambda^{\prime}}{\Delta/2}}\left( {1 + {\frac{\Delta}{2}\lambda^{\prime}}} \right)}}}} & (25)\end{matrix}$

[0111] If a dead zone does not exist, Δ=0 in Equation (25) so that λ=λ′.

[0112] To verify the validity of Equation (25), a test was performed ona first flower garden image complying with the SIF standards. Table 4shows the values of λ for each AC coefficient calculated using Equation(25). It can be seen that the calculated values of λ in the test arevery similar to those shown in Table 3, with the exception that smallerrors occurs in values with respect to several high frequencycomponents. However, these errors can be neglected. TABLE 4 0 1 2 3 4 56 7 0 — 0.018 0.027 0.038 0.055 0.070 0.197 0.169 1 0.016 0.027 0.0350.043 0.069 0.082 0.121 0.178 2 0.021 0.029 0.036 0.046 0.061 0.0830.122 0.191 3 0.025 0.032 0.038 0.047 0.060 0.082 0.153 0.224 4 0.0280.033 0.042 0.051 0.065 0.097 0.177 0.250 5 0.031 0.035 0.044 0.0530.069 0.105 0.196 0.309 6 0.035 0.039 0.045 0.065 0.076 0.133 0.2370.415 7 0.043 0.044 0.051 0.061 0.085 0.177 0.223 0.450

[0113] In the case where a MPEG-1 encoder employs a rate control scheme,a quantization parameter value is assigned an average of quantizationparameters for an entire image to calculate Δ in Equation 6. It isnecessary to estimate the value of λ to use Equation (17), and a delayby one frame occurs when the value of λ is calculated according toEquation (25).

[0114]FIGS. 9A through 9C illustrate an original image, an imagecorresponding to bitstreams transcoded according to a conventionalmethod and an image corresponding to bitstreams transcoded according tothe present invention, respectively.

[0115]FIGS. 10 through 12 are graphs illustrating the results of testinga transcoding method according to the present invention. In a firsttest, simulation was performed on 10 flower garden sequences complyingwith the SIF standards. All frames were encoded in a MPEG-1 intra mode,and the same quantization parameter was applied throughout an image.FIG. 10 shows an average peak signal-to-noise ratio (PSNR) of a sequencewhich was transcoded by a method proposed depending on changes in thevalue of Q_(p). For comparison, an average PSNR of an input MPEG-1sequence and an average PSNR of a sequence which was transcoded by aconventional method are shown together. PSNR gain is about 0.3-0.6 dB.Table 5 shows average numbers of bits generated in the above three case.TABLE 5 Quantization MPEG-1 Transcoded Transcoded parameters coded(conventional) (proposed) Bit saving  2 297509 304626 293659 10967(3.60%)  4 228122 241486 229292 10012 (4.14%)  6 187950 204710 19137813332 (6.51%)  8 159041 177585 165223 12362 (6.96%) 19 138606 158682149244  9438 (5.94%) 12 123148 144156 135114  9042 (6.27%) 14 111746133572 125123  8149 (6.10%) 16 100669 123430 114320  9110 (7.38%) 18 92440 116210 106748  9462 (8.14%) 20  85294 109999 101935  8064 (7.33%)22  29598 105182  97548  7934 (7.52%) 24  74153 100612  93008  7604(7.55%)

[0116] From Table 5, two interesting results can be inferred. One isthat the number of bits generated in the proposed method is 3.6-7.5%smaller than that generated in the conventional method. This result canbe explained based on the distribution of DCT coefficients. DCTcoefficients are subject to a Laplacian distribution, in which a balancepoint C_(m) having a positive value is on the left of the center betweent_(m) and t_(m+1). Since r_(m) is at the center between t_(m) andt_(m+1), C_(m) is usually smaller than r_(m). Accordingly, a probabilitythat a smaller reconstruction level is selected increases. Since a smallreconstruction level is used, the number of bits generated decreases.

[0117] The second interesting result is that the number of bits aftertranscoding is always larger than the number of input MPEG-1 bits. Thisis because for the same quantization parameter, a MPEG-4 quantizationstep size is always smaller than a MPEG-1 quantization step size.Accordingly, MPEG-4 uses a more accurate quantization step size, and asa result, the number of bits generated increases.

[0118] A second test was performed on table tennis test sequences underthe same conditions as the first test. Like FIG. 10, average PSNRs areshown in FIG. 11. Gain according to the proposed present method is about0.2-0.4 dB. In Table 6, the numbers of bits in the three cases asdescribed in FIG. 10 are compared. It can be seen that about 5-7% of thebits conventionally used is saved. TABLE 6 Quantization MPEG-1Transcoded Transcoded parameters coded (conventional) (proposed) Bitsaving  4 172314 197318 186303 11015 (5.58%)  6 124859 152412 143593 8819 (5.78%)  8  98753 125954 118839  7115 (5.64%) 19  80212 105236 97573  7663 (7.28%) 12  67408  90390  84308  6082 (6.72%) 14  58334 72344  67354  4990 (6.89%) 16  52039  65004  61114  3850 (5.98%) 18 45919  58261  54391  3870 (6.61%) 20  51684  53324  49381  3943 (7.39%)22  38170  50124  46381  3743 (7.46%) 24  35613  46312  42785  3527(7.61%) 26  33328  43981  40503  3478 (7.94%)

[0119] A third test was performed on football sequences complying withthe SIF standards. Each football sequence had a GOP whose size is 10 andwas coded at a rate of 1.5 Mbps. FIG. 12 illustrates the PSNRs of 11th,21st, 31st . . . frames which are I-pictures. It can be seen from FIG.12 that the proposed method has gain of about 0.3-0.5 dB as compared tothe conventional method. The number of bits generated according to theconventional method was 195805, and the number of bits generatedaccording to the proposed method was 183731. Accordingly, about 6% ofthe bits was saved.

[0120]FIG. 13 is a block diagram illustrating a transcoding apparatusaccording to the present invention. Referring to FIG. 13, a transcodingapparatus 1300 performs transcoding according to the transcoding methoddescribed with reference to FIGS. 4 through 7B. Thus, the above contentsdescribed with reference to FIGS. 4 through 7B will be referred to fordescribing the detailed operations of members.

[0121] The transcoding apparatus 1300 includes a variable length decoder(VLD) 1302, a MPEG-1 inverse quantizer 1304, a MPEG-4 quantizer 1306, avariable length coder (VLC) 1308 and a MPEG-4 syntax generator 1310. TheVLD 1302 reconstructs syntax elements and a quantized DCT coefficientfrom an input MPEG-1 bitstream and provides them to the MPEG-4 syntaxgenerator 1310 and the MPEG-1 inverse quantizer 1304. The MPEG-1 inversequantizer 1304 inverse-quantizes the quantized DCT coefficient accordingto MPEG-1 to reconstruct a DCT coefficient and provides thereconstructed DCT coefficient to the MPEG-4 quantizer 1306. The MPEG-4quantizer 1306 quantizes the reconstructed DCT coefficient according toMPEG-4 to obtain a quantized DCT coefficient and provides the quantizedDCT coefficient to the VLC 1308. By controlling a quantization step inthe MPEG-4 quantizer 1306, a bit rate can be changed.

[0122] Meanwhile, the MPEG-4 syntax generator 1310 maps the syntaxelements provided from the VLD 1302 to syntax elements complying withMPEG-4 and provides the MPEG-4 syntax elements to the VLC 1308. Themapping operation of the MPEG-4 syntax generator 1310 has been describedin detail with reference to FIG. 4 and Tables 1 and 2.

[0123] The VLC 1308 variable length codes the quantized DCT coefficientprovided from the MPEG-4 quantizer 1306 and the syntax elements providedfrom the MPEG-4 syntax generator 1310 according to the MPEG-4 andoutputs the result. A bitstream output from the VLC 1308 has beencompressed according to the MPEG-4 and has a desired aspect ratio andbit rate.

[0124] A transcoding apparatus according to the present inventionperforms conversion between bitstream compressed according to differentcompression methods. In the above embodiments, a method and apparatusfor converting a MPEG-1 bitstream into a MPEG-4 bitstream has beendescribed, but it will be apparent that the present invention can beapplied to the case of converting a MPEG-2 bitstream to a MPEG-4bitstream and to conversion between bitstreams compressed according tothe same method. In addition, the present invention can be applied tothe case of converting a MPEG-4 bitstream to a MPEG-1 or MPEG-2bitstream.

[0125] As described above, the present invention facilitates theconversion between bitstreams compressed according to different methodsor the same method. Therefore, a transcoding method and apparatusaccording to the present invention allows existing digital videocontents complying with MPEG-1 or MPEG-2 to be used in terminalsdesigned to use MPEG-4 digital video contents such as Internet video,mobile video and smart media.

What is claimed is:
 1. A transcoding method of performing conversionbetween compressed bitstreams having at least syntax elements and videoelements corresponding to video data, the transcoding method comprisingthe steps of: a) decoding a first bitstream compressed according to afirst compression method and parsing syntax elements and video elements;b) mapping the parsed syntax elements to syntax elements complying witha target second compression method; c) partially reconstructing videodata complying with the first compression method from the parsed videoelements; d) requantizing the video data reconstructed in the step c)according to the second compression method; and e) coding the mappedsyntax elements and the requantized video data to obtain a bitstreamcomplying with the second compression method.
 2. The transcoding methodof claim 1, wherein the first compression method is a moving pictureexperts group (MPEG)-1 compression method, the second compression methodis a MPEG-4 compression method, and the step b) comprises: b-1)converting a MPEG-1 f_code into a MPEG-4 f_code; b-2) converting aMPEG-1 macroblock (MB) type into a MPEG-4 MB type; b-3) converting aMPEG-1 coded block pattern (CBP) into a MPEG-4 CBP; and b-4) convertinga MPEG-1 MQUANT value (a quantization parameter in MPEG-1) into a MPEG-4DQUANT value (a difference of quantization parameters).
 3. Thetranscoding method of claim 2, wherein the step b-1) performs theconversion according to the following equation, vop _(—) f_code_forward=max((forward_(—) f_code-1), 1) where max(a, b) is an operator ofselecting a larger value between “a” and “b”.
 4. The transcoding methodof claim 2, wherein the step b-2) comprises the steps of: (i) setting“nomc+coded” as a MPEG-4 “inter” type and setting a motion vector to (0,0); (ii) setting “nomc+coded+q” as a MPEG-4 “inter+q” type and setting amotion vector to (0, 0); (iii) setting “mc+not coded” as a MPEG-4“inter” type, using a motion vector as it is, and setting both “cbpy”and “cbpc” to zero; and (iv) setting the value of “code” determining“not coded” in MPEG-4 to 0 such as “cod=0” as many times as skipped MBs.5. The transcoding method of claim 2, wherein the step b-3) comprisesthe steps of: b-3-1) individually coding cbpy according to the followingequation, cbpy=(cbp&0x3c)>>2 where “&” indicates an AND operationperformed in bit unit, “Ox3c” indicates “3c” of a hexadecimal number,and “>>n” indicates an n-bit right shift operation; and b-3-2) codingcbpc according to the following equation, cbpc=(cbp&0x03)>>2,  and thecbpc is united with the MB type obtained in the above step b-2) andcoded to comply with an mcbpc VLC table of corresponding MPEG-4 I-VOPand P-VOP.
 6. The transcoding method of claim 2, wherein the step b-4)performs the conversion according to the following equation, dquant=min(max((mquant of current MB−mquant of previous MB),−2), 2).
 7. Thetranscoding method of claim 2, wherein the step d) comprises the stepsof: estimating a Laplacian distribution of a discrete cosine transform(DCT) coefficient reconstructed from a MPEG-1 bit stream; determining areconstruction level using the estimated Laplacian distribution of theDCT coefficient; and performing quantization according to MPEG-4 usingthe determined reconstruction level.
 8. The transcoding method of claim2, wherein when an output y with respect to an input DCT coefficient xis expressed by${y = {{Q_{1}(x)} = \left\lfloor {\left\lfloor {\frac{x}{\Delta} + \frac{1}{2}} \right\rfloor \cdot \Delta} \right\rfloor}},a$

quantization step size α_(i) is given by${{\Delta \quad i} = \frac{{Wi} \cdot Q_{p}}{8}},{i = 0},1,{2\quad \cdots}\quad,63$

(Q_(p) is a quantization parameter), a decision level t_(m) is given by${t_{m} = {\left( {m - \frac{1}{2}} \right) \cdot \Delta}},{m \geq 1},$

x_(m)={x|xε[t_(m), t_(m+1)]} when x belongs to a section [t_(m),t_(m+1)], an amplitude level λ_(m) of x_(m) is expressed by${\lambda_{m} = \left\lfloor {\frac{x_{m}}{\Delta} + \frac{1}{2}} \right\rfloor},$

an output x′ with respect to the input DCT coefficient y, which has beenquantized by a MPEG-1 quantizer having a dead zone in which areconstruction level for x_(m), that is, an inverse-quantized DCTcoefficient r_(m) is given by r_(m)=└λ_(m)·Δ┘, is expressed by$x^{\prime} = {{Q_{2}(y)} = \left\{ {\begin{matrix}{\left\lfloor {{\left\lfloor \frac{y}{\Delta^{\prime}} \right\rfloor \cdot \Delta^{\prime}} + \frac{\Delta^{\prime}}{2}} \right\rfloor \quad {if}\quad Q_{p}\quad {is}\quad {odd}} \\{\left\lfloor {{\left\lfloor \frac{y}{\Delta^{\prime}} \right\rfloor \cdot \Delta^{\prime}} + \frac{\Delta^{\prime}}{2}} \right\rfloor \quad - {1\quad {if}\quad Q_{p}\quad {is}\quad {even}}}\end{matrix},} \right.}$

a quantization step size Δ′ is given by Δ′=2Q_(p), a decision levelt′_(n) is given by t′_(n)=n·Δ′, n≧1, y_(n)={y|yε[t′_(n), t′_(n+1)]} whenthe output y belongs to a section [t′_(n), t′_(n+1)], and an amplitudelevel of y_(n) that is, an inverse-quantized DCT coefficient λ′_(n) isrequantized by a MPEG-4 quantizer having a dead zone defined as$\lambda_{n}^{\prime} = \left\lfloor \frac{y_{n}}{\Delta^{\prime}} \right\rfloor$

and is converted into a MPEG-4 DCT coefficient, the step d) comprisesthe steps of: d-1) defining subscript values allowing the decision levelto belong to a section [t_(m), t_(m+1)] as a set p={p|t′_(p)ε[t_(m),t_(m+1)]}; d-2) defining candidates of the subscript values of thedecision level as a set K=P∪{min{P}−1} where the symbol ∪ indicates aunion and an operator min{A} indicates a minimum value among the membersof a set A; and d-3) selecting a member satisfying a cost function fromamong the candidate 22 subscript values as a final subscript value, thecost function being expressed by$k = {{\arg_{k\varepsilon K}^{\min}{{C_{m} - r_{k}^{\prime}}}\quad {where}\quad C_{m}} = \frac{\int_{t_{m}}^{t_{m} + 1}{{x \cdot {p(x)}}\quad {x}}}{\int_{t_{m}}^{t_{m} + 1}{{p(x)}\quad {x}}}}$

where C_(m) is an optimum reconstruction level in the section [t_(m),t_(m+1)] used by a Lloyd-Max quantizer in view of mean square error, andp(x) is a Laplacian distribution function.
 9. The transcoding method ofclaim 8, wherein in the step d-3), C_(m) is obtained by analyzing thestatistical characteristic of p(x).
 10. The transcoding method of claim9, wherein when it is assumed that AC DCT coefficients comply with aLaplacian distribution expressed by${{p(x)} = {\frac{\lambda}{2} \cdot ^{{- \lambda}{x}}}},$

a step of determining the value of λ determining the statisticalcharacteristic of p(x) comprises the steps of: d-3-1) calculating anaverage of a random variable |x| according to${{E\left( {x} \right)} = {{\int_{- \infty}^{\infty}{{{x} \cdot {p(x)}}\quad {x}}} = {{\int_{- \infty}^{\infty}{{{x} \cdot \frac{\lambda}{2} \cdot ^{{- \lambda}{x}}}\quad {x}}} = \frac{1}{\lambda}}}};$

d-3-2) determining λ according to λ=1/E(|x|).
 11. The transcoding methodof claim 10, wherein the step d-3-2) comprises the steps of: d-3-2-1)approximating the value of E(|x|) according toE(|x|)≡E(|y|)+E(|z|)_(Δ/2)  where E(|z|)_(Δ/2)=∫_(−Δ/2)^(Δ/2)|z|·p(z)dz, and p(z)=λ′/2·e^(−λ′|z|) where${\lambda^{\prime} = \frac{1}{E\left( {y} \right)}};$

d-3-2-2) calculating E(|z|)_(Δ/2) according to${\left( {E\left( {z} \right)} \right)_{\frac{\Delta}{2}} = {{2 \cdot {\int_{0}^{\frac{\lambda}{2}}{{z \cdot \frac{\lambda^{\prime}}{2} \cdot ^{{- \lambda^{\prime}}/z}}\quad {z}}}} = {\frac{1}{\lambda^{\prime}} - {^{{- \lambda^{\prime}}{\Delta/2}}\left( {\frac{1}{\lambda^{\prime}} + \frac{\Delta}{2}} \right)}}}};$

 and d-3-2-3) estimating the value of λ according to$\lambda = {{\frac{1}{E\left( {x} \right)} \cong \frac{1}{{E\left( {y} \right)} + \left( {E\left( {z} \right)} \right)_{\frac{\Delta}{2}}}} = {\frac{\lambda^{\prime}}{2 - {^{{- \lambda^{\prime}}{\Delta/2}}\left( {1 + {\frac{\Delta}{2}\lambda^{\prime}}} \right)}}.}}$


12. A requantizing method in which an output y with respect to an inputDCT coefficient x is expressed by${y = {{Q_{1}(x)} = \left\lfloor {\left\lfloor {\frac{x}{\Delta} + \frac{1}{2}} \right\rfloor \cdot \Delta} \right\rfloor}},$

a quantization step size Δ_(i) is given by${{\Delta \quad i} = \frac{{Wi} \cdot Q_{p}}{8}},$

i=0, 1, 2 . . . 63 (Q_(p) is a quantization parameter), a decision levelt_(m) is given by t_(m)=(m−½)·Δ, m≧1, x_(m)={x|xε[t_(m), t_(m+1)]} whenx belongs to a section [t_(m), t_(m+1)], an amplitude level λ_(m) ofx_(m) is expressed by${\lambda_{m} = \left\lfloor {\frac{x_{m}}{\Delta} + \frac{1}{2}} \right\rfloor},$

an output x′ with respect to the input DCT coefficient y, which has beenquantized by a MPEG-1 quantizer having a dead zone in which areconstruction level for x_(m), that is, an inverse-quantized DCTcoefficient r_(m) is given by r_(m)=└λ_(m)·Δ┘, is expressed by$x^{\prime} = {{Q_{2}(y)} = \left\{ {\begin{matrix}\left\lfloor {{\left\lfloor \frac{y}{\Delta^{\prime}} \right\rfloor \cdot \Delta^{\prime}} + \frac{\Delta^{\prime}}{2}} \right\rfloor & {{if}\quad Q_{p}\quad {is}\quad {odd}} \\{\left\lfloor {{\left\lfloor \frac{y}{\Delta^{\prime}} \right\rfloor \cdot \Delta^{\prime}} + \frac{\Delta^{\prime}}{2}} \right\rfloor - 1} & {{if}\quad Q_{p}\quad {is}\quad {even}}\end{matrix},} \right.}$

a quantization step size Δ′ is given by Δ′=2Q_(p), a decision levelt′_(n) is given by t′_(n)=n·Δ′, n≧1, y_(n)={y|y ε[t′_(n), t′_(n+1)]}when the output y belongs to a section [t′_(n), t′_(n+1)], and anamplitude level of y_(n), that is, an inverse-quantized DCT coefficientλ′_(n) is requantized by a MPEG-4 quantizer having a dead zone definedas$\lambda_{n}^{\prime} = \left\lfloor \frac{y_{n}}{\Delta^{\prime}} \right\rfloor$

and is converted into a MPEG-4 DCT coefficient, the requantizing methodcomprising the steps of: d-1) defining subscript values allowing thedecision level to belong to a section [t_(m), t_(m+1)] as a setp={p|t′_(p)ε[t_(m), t_(m+1)]}; d-2) defining candidates of the subscriptvalues of the decision level as a set K=P∪{min{P}−1} where the symbol ∪indicates a union and an operator min{A} indicates a minimum value amongthe members of a set A; and d-3) selecting a member satisfying a costfunction from among the candidate subscript values as a final subscriptvalue, the cost function being expressed by$k = {{\arg_{k\varepsilon K}^{\min}{{C_{m} - r_{k}^{\prime}}}\quad {where}\quad C_{m}} = \frac{\int_{t_{m}}^{t_{m} + 1}{{x \cdot {p(x)}}\quad {x}}}{\int_{t_{m}}^{t_{m} + 1}{{p(x)}\quad {x}}}}$

where C_(m) is an optimum reconstruction level in the section [t_(m),t_(m+1)] used by a Lloyd-Max quantizer in view of mean square error, andp(x) is a Laplacian distribution function.
 13. The requantizing methodof claim 12, wherein in the step d-3), the balance point C_(m) isobtained by analyzing the statistical characteristic of p(x).
 14. Therequantizing method of claim 13, wherein when it is assumed that AC DCTcoefficients comply with a Laplacian distribution expressed by${{p(x)} = {\frac{\lambda}{2} \cdot ^{{- \lambda}{x}}}},$

a step of determining the value of λ determining the statisticalcharacteristic of p(x) comprises the steps of: d-3-1) calculating anaverage of a random variable |x| according to${{E\left( |x| \right)} = {{\int_{- \infty}^{\infty}\left| x \middle| {{\cdot {p(x)}}{x}} \right.} = {{\int_{- \infty}^{\infty}\left| x \middle| {{\cdot \frac{\lambda}{2} \cdot e^{{- \lambda}|x|}}{x}} \right.} = \frac{1}{\lambda}}}};$

 and d-3-2) determining λ according to$\lambda = {\frac{1}{E\left( |x| \right)}.}$


15. The transcoding method of claim 14, wherein the step d-3-2)comprises the steps of: d-3-2-1) approximating the value of E(|x|)according to E(|x|)≡E(|y|)+E(|z|)_(Δ/2) where E(|z|)_(Δ/2)=∫_(−Δ/2)^(Δ/2)|z|·p(z)dz, and p(z)=λ′/2·e^(−λ′|z|) where${\lambda^{\prime} = \frac{1}{E\left( |y| \right)}};$

d-3-2-2) calculating E(|z|)_(Δ/2) according to${{E\left( |z| \right)}_{\frac{\Delta}{2}} = {{2 \cdot {\int_{0}^{\frac{\lambda}{2}}{{z \cdot \frac{\lambda^{\prime}}{2} \cdot e^{{- \lambda^{\prime}}/z}}{z}}}} = {\frac{1}{\lambda^{\prime}} - {e^{{- \lambda^{\prime}}{\Delta/2}}\left( {\frac{1}{\lambda^{\prime}} + \frac{\Delta}{2}} \right)}}}};$

 and d-3-2-3) estimating the value of λ according to$\lambda = {{\frac{1}{E\left( |x| \right)} \cong \frac{1}{{E\left( |y| \right)} + {E\left( |z| \right)}_{\frac{\Delta}{2}}}} = {\frac{\lambda^{\prime}}{2 - {e^{{- \lambda^{\prime}}{\Delta/2}}\left( {1 + {\frac{\Delta}{2}\lambda^{\prime}}} \right)}}.}}$


16. A transcoding apparatus of performing conversion between compressedbitstreams having at least syntax elements and video elementscorresponding to video data, the transcoding apparatus comprising: adecoder for reconstructing syntax elements and video elements from afirst bitstream complying with a first compression method; an inversequantizer for inverse-quantizing the video elements provided from thedecoder according to the first compression method to reconstruct videodata; a quantizer for requantizing the video data according to a secondcompression method; a syntax generator for mapping the syntax elementsprovided from the decoder to syntax elements complying with the secondcompression method; and an encoder for encoding the requantized videodata (video elements complying with the second compression method)provided from the quantizer and the syntax elements provided from thesyntax generator according to the second compression method, therebyoutputting a second bitstream.
 17. The transcoding apparatus of claim16, wherein the first compression method is a moving picture expertsgroup (MPEG)-1 or MPEG-2 compression method, and the second compressionmethod is a MPEG-4 compression method.